ANSI Class VI Valve Seat Leakage 3

[BOS]

Can ANSI Class VI Seat Leakage be represented in terms a percentage drop in pressure?

The use of flowrates & bubbles per minute presents problems in a practical situation....

Suggestions??

 

[BillBirch]

The leakage class represents the tightness at shut-off. The pressure drop percentage would be dependant upon whether the contained fluid is liquid or gas and the volume contained. Essentially the rating is an indication of the quality of manufacture, as indicated by the valve's shut-off performance. No control performance should be inferred by the rating.

 

[valvetech]

There is a general misunderstanding that Class VI shutoff means no leakage through the valve seats when closed. Nothing could be further from the truth. The standard does not state that there will be no leakage under any conditions. To the contrary it defines how much leakage is allowed under a very specific set of conditions.
ANSI/FCI 70-2 standard for allowable seat leakage spells out exactly how leakage teset are to be administered, and exactly how much seat leakage is allowed under those very precise conditions. For instance, a valve for Class VI shutoff is tested in the factory on air at 50 -125 degrees F at 50 psi, and if the seat leakage is less than one bubble in one minute, it will pass the test.
If we put a valve in the field that passed the factory test for Class VI shutoff in an actual process with natural gas at 300 psi at 200 degrees F will that valve pass more than one bubble per minute? The answer is yes. Does that valve meet the Class VI shutoff standard? Yes again.
How many real systems in the field operate at the test conditions? If you answered "Not very many" you are correct.
It is important when specifying a valve to be aware of how the system's seat leakage requirements relate to the standard.

 

The valve leakage can be less at higher pressures if the seats are pressure-energised.

 

[NGiLuzzu]

I agree with "valvetech". Test conditions are considered to be "ideal", so that pressure drop is not foreseen.
In addition, if you look at the European standard IEC 534, part 4, amendment 1 (pratically equivalent to ANSI/FCI 70-2), you can find how to extend Class VI to higher pressures and larger diameters than those in the standards themselves... but remember this is just an extrapulation!!
Hope this can be useful, 'NGL